Flexography (or flexo) is a direct rotary printing method that uses relief plates made of photopolymer materials. The plates are flexible and soft, hence the name flexography or flexo. These plates are inked and the print is obtained by direct deposition of the ink onto the media to be printed thanks to a light pressure exerted by a printing cylinder on which the plates are placed.
Flexography is a high-speed printing process, capable of printing on many types of absorbent and non-absorbent materials. Some typical applications of flexographic printing are in the manufacture of paper and plastic bags, milk cartons, disposable cups and the like, however thanks to developments in the print quality, flexographic printing is now also used for the printing of newspapers, for example, as well as of envelopes, of labels, and in printing on plastic, films and plates of acetate, wrapping paper and for many materials used in the packaging of products.
As is known, the prepress step, namely the preparation of the flexographic plate which undergoes various steps, among which is the exposure step of the plate to ultraviolet (UV) light for the polymerization thereof, is very important and delicate.
In order to obtain the desired relief characters, the plate is only polymerized at the points of a mask left permeable to the light. In the case of the so-called analogue printing plates, a negative film plate is applied to the plate, which indeed shows the image to be printed in negative, while in the case of the so-called digital printing plates, the plate itself is covered by a layer that is abraded by laser engraving to create the negative of the image to be printed.
The exposure of the plate to light causes the polymerization of only those parts of the plate at the points of the negative film that are permeable to the light, in the case of an analogue printing plate, or at the points in which the layer of coating material has been removed in the case of a digital printing plate, thus forming the image to be printed in relief on the polymer.
In any case, the plate must be exposed to ultraviolet (UV) light for the necessary formation of the base (“back” exposure, i.e. of the bottom or back of the plate). In general, with the known apparatuses of the prior art, the main exposure (i.e. of the main side of the plate) and back exposure steps take place in separate steps of the printing plate treatment process.
Subsequently to the exposure step, the flexographic plate undergoes other processing steps, among which the washing step, thanks to which the unpolymerized material is removed from the plate.
According to what is known in the state of the art, the washing step is performed successively to the exposure step of the plate and using a special washing module. The operator must therefore manually move the printing plate from the exposure module to the washing module, by in general removing the plate from the exposure module and repositioning it at the entrance to the washing module.
The washing of the plate can be performed by means of various known techniques, by means of solvent that penetrates and dissolves the non-photopolymerized polymer or by means of water that detaches, does not dissolve, the non-photopolymerized polymer, and subsequent removal of the non-photopolymerized by means of brushes.
Again, according to what is known in the prior art, the washing step thus provides that the plate be made to slide on a work surface surmounted by a series of brushes set in motion at a pre-determined distance from the work surface. The plate is fed at the speed calculated for optimal removal of the polymer: the greater the engraving thickness of the polymer to be removed, the lower must be the feed speed of the plate in the washing station, and therefore the greater the washing time.
To provide some examples, typical plate thicknesses are for example:
total thickness 1.14 mm, engraving thickness 0.6 mm;
total thickness 6 mm, engraving thickness 3 mm.
The feed speed of the plate in the washing station not only depends on the thickness of the plate, but also on the number of consecutive brushes working the plate during washing.
Again, so as to provide some examples, a plate having a thickness of 1.14 mm can be washed with a feed speed of about 200 mm/min, while a plate having a thickness of 6 mm will be washed at a speed of about 40 mm/min, with a overall washing time that is longer and dependent on the size of the plate.
In consideration of the intrinsic characteristics of the various preparation steps of the printing plate, in particular, the exposure and washing steps considered herein, in the prior art these are carried out, as previously mentioned, in separate machines or modules.
In particular, the printing plate is rested, generally manually, on the surface of an exposure unit and exposed to the light by setting exposure time and radiating power according to what is known from experience. The exposure time does not vary much on variation of the thickness of the plate, while it is dependant on the intensity of the radiated power in the time unit, measured in watts or joules/second. With a flat exposure unit it is possible to perform main and back polymerization in the same module, while other solutions known in the prior art provide that the plate be supported by a roller during the main exposure steps, which naturally makes it impossible to also contemporaneously perform the back exposure of the plate.
The photo polymerized plate is then extracted from the exposure unit; as previously mentioned, this operation is generally performed manually and inserted into the machine or washing station for the subsequent washing operation.
In the sector there is a strong need for the possibility of increasing the speed and efficiency of the preparation steps of the flexographic plate by automating them so that the operator does not need to manually move printing plates that can also be 2 meters to 1.3 meters in size, and so as to have processes suitable for the continuous processing of a large number of printing plates.